The invention relates generally to method and laser apparatus for treating glaucoma in a human eye. In particular, the invention relates to a method and laser apparatus for treating open angle glaucoma using a gonioscopic laser trabecular ablation procedure.
Open angle glaucoma is a potentially debilitating disease of the eye which, if left untreated, may lead to blindness. While the cause for open angle glaucoma is not well understood, all existing treatments are aimed at lowering intraocular pressure to slow or arrest the progression of the disease. Intraocular pressure reduction can be achieved using drugs. However, drugs are often not effective or their effect diminishes over time. When this happens, various surgical procedures are performed to lower intraocular pressure.
These surgical procedures are generally aimed at either decreasing the production of the aqueous humor or increasing the outflow facility in the eye. Examples of the former includes cyclo-destructive procedures which are aimed at killing a percentage of the ciliary processes that produce aqueous humor in the eye. Examples of the latter include procedures such as trabeculectomy and laser trabeculoplasty.
With respect to the latter procedures, it has been shown that the site of the greatest outflow resistance is at the trabecular meshwork. The trabecular meshwork is a fibrous structure situated at the edge of the cornea in the region of the anterior chamber known as the angle. The aqueous humor drains through the trabecular meshwork into Schlemm's canal. Investigators have experimented with various surgical procedures aimed at creating holes in the meshwork, extending into Schlemm's canal, postulating that outflow facility would be improved.
For example, in the 1970's, an investigator used a Q-switched ruby laser to create holes in the trabecular meshwork. The pulses of laser energy were delivered using a goniolens, and the pulse duration was on the order of 30 nanoseconds. For this procedure, the mechanism for hole creation is optical breakdown, which occurs at the focus of high peak power laser beams. The optical breakdown creates a mechanical shockwave which then mechanically tears a hole in the meshwork. This procedure did achieve significant intraocular pressure reduction in a large percentage of the subjects. However, a problem with the procedure was that the pressure-lowering effect lasted only about three months on average.
Other investigators studied a similar procedure using Q-switched Nd:YAG lasers with pulse durations in the 10 nanosecond range. Similar results to the ruby laser described above were achieved for primary open angle glaucoma. However, only a subgroup of subjects suffering from juvenile glaucoma benefited over the long term from this treatment.
More recently, another group of investigators proposed using mid-infrared (i.e., a near 3 micron wavelength) laser radiation to ablate holes in the meshwork. The goal of this procedure is to create holes extending directly into Schlemm's canal. However, because the 3 micron energy is strongly absorbed by water which is prevalent in the eye, the laser energy must be delivered by a fiber that is physically brought into contact with the trabecular meshwork. In other words, the procedure is invasive. It is speculated that because of the strong water absorption of the laser radiation, a clean ablation hole can be created in the meshwork with minimal collateral thermal damage. A significant disadvantage of the proposed procedure is the invasive nature of the treatment.
Laser trabeculoplasty is another accepted procedure for the treatment of open angle glaucoma. This procedure is commonly performed using either a cw argon laser or a cw diode laser. In this procedure, the laser energy is delivered gonioscopically using a contact goniolens. The laser energy is focused into a 50-100 micron diameter spot on the trabecular meshwork. The laser energy heats the meshwork tissue until a white spot (or laser burn) is created. Over 100 such spots may be created over the entire 360.degree. area of the meshwork. Interestingly, no meshwork tissue is actually ablated or removed, and no holes are created in the meshwork. This procedure has been effective in lowering intraocular pressure in over 80% of the subjects.
Although the exact mechanism which causes the reduction of intraocular pressure in laser trabeculoplasty has never been fully elucidated, it is generally accepted that the laser burns cause shrinkage in the trabecular meshwork tissue. This shrinkage is believed to cause the meshwork tissue disposed between the laser burns to stretch and become more open, thereby decreasing the resistance of the trabecular meshwork to aqueous outflow.
A problem with laser trabeculoplasty is that the intraocular pressure lowering effect tends to disappear over time. In approximately 50% of the subjects treated with this procedure, intraocular pressure can be adequately controlled for less than about four to five years. For subjects subjected to laser trabeculoplasty and having an intraocular pressure that can no longer be adequately controlled, repeated laser trabeculoplasty is of limited value. In such cases, the only viable alternative is filtration surgery.
Another problem commonly encountered with laser trabeculoplasty is the phenomenon of pressure spiking, where in the days immediately following the trabeculoplasty procedure, the subject's intraocular pressure rises above pretreatment levels. This spiking requires careful monitoring and control with drugs.
It is therefore a principle object of the present invention to provide a non-invasive method and laser apparatus for treating open angle glaucoma using a gonioscopic laser trabecular ablation procedure which results in long term intraocular pressure control.
It is another principle object of the present invention to provide a non-invasivemethod and apparatus for treating open angle glaucoma for subjects who utimately failed laser trabeculoplasty treatment.